# coding:utf-8

import time

def req_a():
    print("start calling a")
    time.sleep(2)
    print("end a")


def req_b():
    print("start b")
    print("end b")


def main():
    """模拟主循环调度者"""
    req_a()
    req_b()

if __name__ == "__main__":
    main()